MICROSCOPIC ILLUSTRATIONS. 281 



exhibit the two or more polarizing axes, where they exist, 

 as spots of light, surrounded by halos, &c. 



The polarization of lenses of gem may always be seen, 

 by trying them on a bat's or mouse's hair, which will be 

 sure to give more than one image, where it exists ; crys- 

 tallizations and flaws are best seen by examining them as 

 microscopic objects with oblique candle-light; they more- 

 over produce a muddiness in the vision of all objects, but 

 are not necessarily associated with polarization. 



CENTERING OF LENSES. 



The centering of a lens may be suspected of inaccuracy 

 when it is impossible to bring an object-glass into which 

 it enters as a constituent part into adjustment, (supposing 

 that there is no fault in the turning of its setting) ; in 

 which case its component annuli will not be concentric 

 with each other, nor will the lucid point at the centre of 

 the disc be truly in the middle of it, but give an appear- 

 ance as at , 14. Convex lenses are seldom ill-centered, 

 but concaves frequently are. The former, in this state, 

 have not their maximum of thickness nor the latter 

 theirs of thinness at the centre. 



ADJUSTMENT OF OBJECT-GLASSES AND METALS. 



Bad centering may be confounded with, or mistaken 

 for, bad adjustment, and vice versd. When an object- 

 glass is out of adjustment, its component lenses are not so 

 posited in their setting that their axes shall be in one right 

 line, which must moreover coincide with the axis of the 



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